Stabilization of polyphosphate fertilizer solutions

ABSTRACT

A minor excess of fluoride over that required to react with all the aluminum and magnesium in ammonium and potassium polyphosphate liquid fertilizer solutions will prevent the precipitation of these elements. Effectively, this addition of excess fluoride shifts the solution composition from a region where the water-insoluble precipitate MgAl(NH4)5(P2O7)2F2.6H2O (I), is stable to a region where the water-soluble form, dimorph II, is stable; thus the metallic cations are sequestered.

United States Patent [191 Frazier Jan. 21, 1975 STABILIZATION OFPOLYPI-IOSPIIATE FERTILIZER SOLUTIONS Inventor: Alva W. Frazier,Florence, Ala.

Tennessee Valley Authority, Muscle Shoals, Ala.

Filed: Oct. 13, 1972 Appl. No.: 297,246

Related U.S. Application Data Continuation-impart of Ser. No. 141,366,May 7, 1971, Pat. No. 3,711,268, which is a continuation of Ser. No.82,809, Oct. 21, 1970, which is a continuation-in-part of Ser. No.30,264, April 20, 1970, abandoned.

Assignee:

U.S. Cl. 71/34, 71/64 C Int. Cl C05b 7/00 Field of Search 71/34, 64 C,35, 36, 1

References Cited UNITED STATES PATENTS 3/1962 Matiinson 71/34 10/1962Reusser et a1. 71/43 RECOMMENDED CALCULATED SE.

3,088,819 5/1963 Funkhouser 71/36 X 3,290,140 12/1966 Young 7l/64 C X3,697,247 10/1962 Jones 71/34 OTHER PUBLlCATlONS Frazier, Def. Pub. ofSerial No. 82,809-h led 10/21/70, published in 890 O6 981 on9/2l/71-Def. Publ. N0. T890,010, 71-34.

Primary ExaminerSamih N. Zaharna Assistant ExaminerRichard Barnes [57]ABSTRACT A minor excess of fluoride over that required to react with allthe aluminum and magnesium in ammonium and potassium polyphosphateliquid fertilizer solutions will prevent the precipitation of theseelements. Effectively, this addition of excess fluoride shifts thesolution composition from a region where the waterinsoluble precipitateMgAl(NH (P O,) F -6H O (l), is stable to a region where thewater-soluble form, dimorph 11, is stable; thus the metallic cations aresequestered.

1 Claim, 1 Drawing Figure l I 20 4o 60 so FRACTION, '7, P205 PRESENT AsPOLYPHOSPHATE EFFECT OF POLYPHOSPHATE ON VALUE OF S.R.

CALCULATED $.R.

RECOMMENDED 0 I E l l 2O 4O 6O 8O FRACTION, I, P 0 PRESENT ASPOLYPHOSPHATE EFFECT OF POLYPHOSPHATE ON VALUE OF S.R.

STABILIZATION OF POLYPHOSPIIATE FERTILIZER SOLUTIONS This application isa continuation-in-part of my copending application Ser. No. 141,366,filed May 7, 1971, now US. Pat. No. 3,711,268 which, in turn, is acontinuation of Ser. No. 82,809, filed Oct. 21, 1970 (now DefensivePublication No. T890,0l); which is a continuation-in-part of Ser. No.30,264, filed Apr. 20, 1970 (now abandoned); all for STABILIZATION OFPOLYPHOSPI-IATE FERTILIZER SOLUTIONS.

The invention herein described may be manufactured and used by or forthe Government for governmental purposes without the payment to us ofany royalty therefor.

My invention relates to a new and improved method for the stabilizationof ammonium polyphosphate liquid fertilizer solutions; particularly itrelates to the prevention of precipitation principally of magnesium andaluminum salts in liquid ammonium polyphosphate fertilizer solutionsprepared from the ammoniation of acid of the wet-process type; and stillmore particularly it relates to the prevention of precipitation ofmagnesium and aluminum salts, i.e., the sequestration thereof by amethod of complexing ions of magnesium and aluminum salts present inammonium polyphosphate solutions from the ammoniation of wet-processphosphoric acid.

Liquid mixed fertilizers having compositions similar to those ofstandard dry-mixed fertilizers are well known in the industry and areever increasing in popularity. Such solutions have the advantage overdry mixed fertilizers in that costs of evaporating water and bagging areeliminated and, perhaps even more importantly, the application to soilis greatly simplified over the use of solid fertilizers and eliminatesdifficulty due to segregation and caking often encountered in storage ofdry fertilizers. I-Iowever, liquid fertilizers have in the past had someoutstanding disadvantages. Rawmaterial costs have, up until the adventof wet-process phosphoric acid, been'relatively high and the solutionsproduced, in many instances, have not been able to compete economicallywith solid fertilizer products. Solutions in the early development offertilizer solutions were limited to a maximum plant food content ofabout 33 percent by weight in that in concentration thereabove thereoccurred unwanted and undesired crystallization and precipitation ofsalts. One of several recent and perhaps the most significantbreakthroughs in overcoming these disadvantages in liquid mixedfertilizers in taught and described in US. Pat. No. 2,950,961, Striplinet al. Therein Striplin teaches that he is able to prepare a liquidmixed fertilizer solution containing substantial values of the primaryplant nutrients in a process wherein he ammoniates high puritysuperphosphoric acid under controlled conditions. It is perhapsnoteworthy that although Striplins teachings appear to be mostlyconcerned with the ammoniation of superphosphoric acid of the furnacetype, he does not appear to be completely limited thereto in that hestates the source of his superphosphoric acid could be from any numberof means or processes including the evaporation of water fromorthophosphoric acid and presumably could be of either the wet orfurnace type. Albeit this is taught in Striplin, there is claimedanother breakthrough in overcoming the disadvantages in liquid mixedfertilizers in application Ser. No. 835,377,

Getsinger, assigned to the assignee of the present invention, and in US.Pat. No. 3,044,851, D. C. Young. The improvements disclosed thereingenerally relate to the production of superphosphoric acid of thewetprocess type by a separate heating and concentrating step in whichthe starting acid is orthophosphoric. The heat-treated acid isconcentrated and condensed to bring about the in situ formation ofpyrophosphoric, tripolyphosphoric, and higher species of polyphosphoricacid, such as to render the acid product similar in many ways to what isnow commonly referred to as furnace superphosphoric acid, the principaldifference being in the amount or degree of impurities contained in themore impure wet super acid, which impurities are principally iron andaluminum, together with magnesium and other congeneric impuritiesleached from the phosphate rock in the preparation of these acids. Instill another fairlyrecent breakthrough in overcoming some of thedisadvantages of producing liquid fertilizer solutions by the prior-artmethods, there is found in US. Pat. No. 3,382,059, Getsinger, theteaching of completely eliminating the separate heating andconcentration step for ortho wet-process phosphoric acid by a directprocess which utilizes in a unique manner the heat of reaction fromammonia for directly producing ammonium polyphosphate solutions, melts,or suspensions from ortho wet-process phosphoric acid.

My invention is not directed primarily to the production of ammoniumpolyphosphate solutions principally derived from the ammoniation ofwet-process phosphoric acid, but rather it is directed to an improvementin stabilizing such ammonium polyphosphate solutions before or afterthey are produced, and my invention is directed specifically to suchsolutions which are derived, in whole or in part, from wet-processphosphoric acid. In the production of such solutions the grade of thefinished material is kept within the range from about 8-24-0 to about11-37-0, and if all conditions for manufacturing and processing thereofare carefully controlled by now known prescribed methods, the resultingsolutions will sometimes remain, upon subsequent storage and handling,as true solutions without any discernible or significant precipitationof salts therein. However, when the finished grade of these solutionscontains greater than about 0.1 percent MgO, for instance about anaverage value of 0.4 percent MgO (oftentimes resulting from the desiroususe of less expensive phosphate rock raw material), the industry hasthen experienced the unfortunate precipitation of principally magnesiumand aluminum salts in such liquid ammonium polyphosphate fertilizersolutions prepared from wet-process phosphoric acid. This problem is aserious one in the fertilizer industry and is encountered at all processand handling points between the manufacturer and the farmer applying thefertilizer in the field. The manufacturer encounters plugged valves,lines, and storage tanks; however, at this point, the required technicalabilities are usually available to circumvent the problem at hand.Still, however, drastic measures may be required at these points ofprocessing, as for example, excessive sediments in the storage tanks ofthe manufacturer can sometimes be removed only by means of employingexplosive action to dislodge the sediments thereby oftentimes weakeningthe storage tank structure. It is, however, after the product leaves themanufacturer that the lack of technically trained personnel complicatesthe precipitation problems several fold. For example, strong acidsolutions cannot be used to dissolve these precipitates which haveplugged river barges or railroad tank cars. Likewise, the sediment inthese instances usually arent deep enough to be susceptible to removalby explosive.

In my earlier work, I have helped to solve this specific problem ofprecipitates formed in ammonium polyphosphate solutions derived in wholeor in part from the ammoniation of wet-process phosphoric acid byshifting the solution compositions to a mildly acidic condition throughthe use of phosphoric acid until the solid phases representing theprecipitates are no longer stable and will dissolve slowly withoutdamage to the tank cars. However, these results of my earlier work haveproven to be time-consuming and require the acquisition and carefulhandling of phosphoric acid, which is usually not readily available ateither the distribution centers or to the farmers, who have the sameprecipitation problems in their equipment as do the manufacturers.

I have now developed a new, novel, and unique method for stabilizingsuch ammonium polyphosphate fertilizer solutions, near neutral pHconditions, derived in whole or in part from wet-process phosphiric acidand which are produced at concentrations or grades wherein theaforementioned precipitation problem occurs. These problems can now bealleviated through exploitation of my discovery that fluorine, as eitherfluoride or fluosilicate, is an efficient complexing agent for thecations in these solutions responsible for precipitate formation andthat this complexing agent will prevent the precipitation when added toor maintained at the proper concentration in such liquid fertilizersolutions.

In my work I have found that the most common precipitate in typicalammonium polyphosphate liquid fertilizers made from wet-process acidproducts of low fluoride content is a crystalline salt with the chemicalcomposition of MgAl(NH (P O F .6H O (I) (water-insoluble dimorph). Incompletely defluorinated wet-process acid products, the compound thatprecipitates is Mg(NH,,) P O,.4H O (orthorhombic dimorph). The amount offluoride remaining in wet-process acid products prepared in the usualmanner frequently is insufficient for precipitation of all the magnesiumas the complex fluoride compound, so that some Mg(NH.,) P O .4H O (O) isprecipitated also. Other precipitates containing magnesium or aluminumthat are known to occur in these fertilizer solutions, but are foundless often are Mg(NH.,) P O i4H O (monoclinic dimorph), Mg(NH,) (P O,).6H O, MgNH.,PO,.6H O, and Al(NH.,) P O OH.3H O.

From these observations, therefore, I became aware that the problem ofsolid-phase formation associated with these liquid fertilizers centersaround the key impurities magnesium, aluminum, and fluorine. A seriouscomplicating factor that obscures their importance when these productsare initially produced is that these precipitation processes require anincubation period before troublesome crystallization begins. Thus,solidphase precipitation problems belatedly occur at unexpected pointsafter production and during handling, distribution, and fieldapplication. The typical conditions that arise due to delayedprecipitation are plugged valves, sludge buildup in storage tanks, riverbarges, or railroad tank cars, and formulation and handling problems atdistribution centers. corrective measures are extremely difficult tocarry out at distribution centers or during field application becauseheretofore about the only effective method has been to adjust the pH ofthe solution to a low value, since acidic solutions are necessary todissolve these water-insoluble compounds. This problem has beenconsidered in the prior art and only two basic processes seem to havebeen proposed to decrease the severity of the problem. In one process,the wet-process phosphoric acid is fully defluorinated" 2 by heatingbefore ammoniation, thus preventing the formation of MgAl(NH (P O F .6HO (I) (waterinsoluble dimorph) which otherwise crystallizes readily asit has a relatively short nucleation period. The fluoride-freefertilizer liquid is still capable of forming unwanted precipitates, butthe only mitigating circumstance here is that these precipitates usuallyrequire a longer time for crystallization to begin. Thus, if theproducts are produced and used within a relatively short time (2 to 6weeks), the chance that a troublesome precipitate will form is small.Suzuki, Y., and Homma, K., Removal of Fluorine from Phosphoric Acid,Japanese Pat. No. 6,817,416, July 23, 1968. 2 Shearon, G. B., andStevenson, G. L., Defluorination of Phosphoric Acid, US. Pat. No.3,429,663, Feb. 25, 1969.

In the other process, the liquid product prepared from wet-process acidis diluted with about one-half its volume of the more expensive ammoniumpolyphosphate product prepared from high purity, electricfurnace gradesuperphosphoric acid, a material that contains no magnesium or aluminumimpurities. This blending dilutes the impurities contributed by thewetprocess acid to a lower degree of supersaturation so that theincubation period required for initiation of precipitation islengthened. Here again, however, there is still the risk of delayedprecipitation, so that the product must be consumed within a limitedstorage time. This second process, however, is somewhat superior to thefirst because the dilution of any wet-process acid product will decreasethe total amount of possible precipitate by one-third. Both processessimply attempt to circumvent or decrease the severity of this problem bylowering the content of magnesium, aluminum, or fluorine impurities.Another associated process which has been accepted to circumvent thisproblem is the production of suspension fertilizer fluids." 6 In thisprocess, higher grade fertilizers are obtained by suspending solidphases which are in equilibrium with the liquid phase in such a mannerthat precipitating impurities do not interfere with the characteristicsof the fertilizers. For example, the impurities of magnesium, aluminum,and fluoride always precipitate in these products as very small crystalswhich are readily suspended when compared to the crystalline phases ofthe primary fertilizer materials, such as KCl, KNO (NHJ HPO and- /or NHH PO Again, however, this process has its inherent problems due toexcessive crystal growth of the primary nutrient materials to a sizewhich can no longer be suspended and also due to the breakdown of thesuspending agents which are available for this purpose.

Striplin, M. M., Jr., Stinson, .I. M., and Wilbanks, J. A., U.S, Pat.No.

3,015,552, Jan. 2, I962.

' Silverberg, J., and Walters, H. K., Com. Fertilizers, 108 (4), 26-7,

66-7, April I964.

5 Slack, A. V., and Nason, M. C., J. Agr. Food Chem., 9 343-8 (Sept-.-Oct. 1961) Liquid Fertilizers from Wet-Process Phosphoric Acid.

Suspension of Impurities.

Slack, A. V., Farm Chemicals, I28 (5), 21-2, 24, 26, May 1965.

It is therefore the principal object of the present invention tostabilize ammonium polyphosphate fertilizer solutions derived in wholeor in part from wet-process phosphoric acid and of grades up to about11-37-0 wherein the formation of undesirable precipitates isencountered, principally caused by the cations of magnesium and aluminumsalts by the controlled addition thereto of fluorine, as either fluorideor fluosilicate, as an efficient complexing agent for these ions tothereby render them unavailable for post-precipitation in the stabilizedsolutions.

I have discovered that the foregoing and other objeets of the presentinvention can be obtained by the addition of fluoride to these samesolutions in moderate amounts over and above the amount already presentto prevent the formation of these salts by shifting thesolutioncomposition to a region in which only soluble complexes or theirwater-soluble salts are stable and can be maintained in solution,namely, the crystalline compounds MgAI(NI-I (P O-,) F .6H O (II)(watersoluble dimorph), Mg(NH (P O .6H O, and (NI-I AlF In carrying outthe objects of my invention in the principal forms thereof, I have foundthat, for my process to be effective, the fluoride content must be heldwithin a certain concentration range. If the fluoride exceeds theprescribed amount, precipitation of these water-soluble salts thenbecomes possible, thereby creating a problem similar to that experiencedwhen the fluoride content is too low. On the other hand, additions offluoride to a concentration below the prescribed level only increasesthe unwanted precipitation problem by forming more MgAl(NH (P O-,) F .6HO (I) until one of the requisite constituents (MgO, M or F) has beencompletely removed from solution. This discovery of an optimum range offluoride or fluosilicate content which gives maximum complexingproperties offers a process by which these products can be essentiallystabilized or clarified of magnesium and/or aluminum precipitates thatmay otherwise separate. The amount to be added has been established byexperimental tests and may be determined by an empirical computation.

Several factors that control the fluoride sequestration of magnesium andaluminum by formation of soluble complexes have been tested in theseexperimental solutions. The most significant test results are summarizedin table I, infra. The samples used for these studies were prepared fromstock ammonium polyphosphate solutions along with other reagents to givedifferent concentrations of MgO, Al O F, and polyphosphate and a rangeof pH values. The mixtures were then seeded with the undesirablecompound, MgAl(NH.,) P O F .6H O (I) and allowed to equilibrate at 25 C.at least 3 weeks before being filtered. The precipitates were. examinedmicroscopically and the filtrates were analyzed chemically. Thecompositions of the clear filtrates then were used to determine theextent to which the fluoride could sequester the magnesium and aluminum,thus giving a liquid fertilizer composition that would be stable orcould precipitate only negligible amounts of solids at this temperature.The table includes the conditions of each test, the composition of theliquid phase, and the stable solid phases found for these solutioncompositions.

A simple mathematical relationship has been devised to express theseresults in terms of a sequestration ratio (S. R.) which is herebydefined as a measurement to show the extent to which fluorine cancomplex magnesium, aluminum, and iron, if present, in a particularsolution composition. This value based on the data available for myoriginal application was previously expressed as S.R. wt.% A1 0 wt.% MgO0.33 wt.% Fe O /wt. F

where the optimum value for SR. was between 1.3 and 1.6. Additionaldata, now included in table I infra, shows that much more fluoride isneeded to sequester aluminum than magnesium. Also, in order that the SR.value will be directly proportional to the quantity of requiredfluoride, the revised formulation is expressed as S.R. %F/wt. MgO+ 3 wt.A +0.33 wt. F8 0 and the optimum value for SR. is now 0.4 to 0.5 forproducts having 50 percent of the total P 0 as polyphosphate.

My most recent work indicates that the factor for A1 0 in the formulajust supra is better set at 2.5 rather than 3.0. See FertilizerSolutions, July-August 1972, pages 32-38 and page 43, and Example VIII,infra.

Since typical 10-34-0 or 11-37-0 fertilizer solutions have pH valuesclose to 6.0 and approximately 50 percent of their P 0 content aspolyphosphate, these conditions were used for most of the examples givenin table 1, except when these variables themselves were being tested, asshown by the footnotes. In examining these results, it must beremembered that the magnesium and aluminum contents in thepost-precipitated, initial liquid products are lowered to as little as0.01 percent MgO and 0.03 percent Al O by the precipitation of MgA]('NH(P O F .6l-I O (I) so that higher concentrations represent cations thathave been sequestered. In the absence of fluoride, Mg(NH P O .4H O andAl(NH P O OI-I.3H O are the troublesome precipitates. For ammoniumpolyphosphate products prepared exclusively from wetprocess acids thathave low magnesium 0.1 percent) and fluoride 0.I percent) contents, theamount of precipitate is not a problem provided the aluminumconcentration is no more than about 1 percent A1 0 However, the currentuse of phosphoric acids reclaimed after use for pickling aluminum metalyields products with abnormally high aluminum contents. Excessive sludgeformation results from the use of these acids with resulting formationof Al(NH P O OH.3H,O, in amounts that make it impossible to handle theproduct with conventional liquid-distributing equipment. Even inammonium polyphosphate liquids such as those made from reclaimed acids,the addition of fluoride was effective in clarifying or redissolving thealuminum precipitate.

Storage temperatures also may influence precipitation, but preliminaryresults indicate that no further crystallization occurred when thesetreated solutions were stored at 0 C. for 4 weeks. The solid phases thatwould be expected to form on cooling for longer storage periods arestill the water-soluble salts mentioned earlier; these would, of course,redissolve on warming to the original temperature. However, underconditions where solids may have formed at low temperatures and meansare not available for reheating these products, the solids could beremoved readily from clogged fertilizer equipment or storage tanks bysimply dissolving them in water. This type of cleanup process cannot beused to dissolve the water-insoluble salts which are the current problemin ammonium polyphosphate liquid fertilizers.

The FIGURE, referred to more specifically in Example VIII, infra,depicts operable and optimum S.R. values of products over a range ofabout 10 to 80 percent of the total P as polyphosphate.

In order that those skilled in the art may better understand how thepresent invention can be practiced, the following examples of myimproved method for stabilizing ammonium polyphosphate solutions againstthe aforementioned undesirable precipitation are given by way ofillustration only and not by way of limitation, and more particularlythe following examples are offered principally to show the extent of thecomplexing power of fluoride on magnesium, aluminum, and iron inammonium polyphosphate liquid fertilizers. The composition range for %Mg0 and %Al O in these tests were chosen to more than cover the valuesusually found in typical products, the average concentrations of whichare about 0.5 percent MgO and 1.0 percent A1 0 These values do notnecessarily limit the compositions at which fluoride will sequesterthese cations.

EXAMPLE I Table I the SR. values and polyphosphate levels for this firstgroup of samples in table I infra. These results show that polyphosphateunits over and above that required for sequestration of the iron andaluminum in liquid fertilizer are an uneconomical luxury which ismagnified by the fact that an increasing quantity of fluoride is thenrequired to sequester the magnesium.

EXAMPLE II Effect of pH Sample Group B, Table 1, supra, shows that thepH has an insignificant effect on the sequestration ratio. A slightlyhigher S.R. value is obtained at pH 6.5 as compared with that at pH 6.0,but it is still well below the recommended value 0.4 to 0.5 for percentpolyphosphate products.

EXAMPLE 111 Effect of Fluoride Source The pair of samples in Group C,Table I, supra, show that fluorine as ammonium fluosilicate willsequester magnesium and aluminum as well as or better than fluorine asammonium fluoride. This effect is significant since the fluosilicate ioncan be obtained at low cost at fertilizer plants by scrubbing theoffgases from either electric-furnace or acidulation processes fortreatment of phosphate rock. Likewise, other soluble sources of fluoridehave been tested at a S.R. value of 0.4 and have been found to beequally effective, for example, KF, K SiF KI-IF NI-I I-1F NaF,Na1-IF ,NaSiF ,1-IF, and

Sequestration by Fluorine of Impurities in Ammonium PolyphosphateFertilizer Solutions (Tests made at pH 6.0 and with equal amounts oforthoand pyrophosphate P 0 F added as NH F) Composition, 72. ofSequestration Mixture clarified solution ratio S.R. Group No. MgO A1 0 FP 0 Fe O Solid phase A 51 0.51 0.70 0.90 37.0 (NH A1F .34 l 0.51 0.460.78 37.1 (NHJ AIF .41 18' 0.90 0.90 1.53 34.5 (NH );AlF,+MgA1(l) .43 30.70 0.40 0.25 34.0 None .12 B 1H 1.70 1.80 2.50 34.2 MgA1(11) .35 2H1.45 1.50 2.10 35.0 MgAl (I1) .35 3H 0.55 0.56 0.82 31.0 MgA1(ll) .37 2H1.45 1.50 2.10 35.8 MgA1(1I) .35 1S 1.75 1.80 2.30 38.8 MgA1(ll) .326513 1.50 1.00 2.90 35.0 MgN .64 563 1.60 0.45 3.60 35.4 (NH );,A1F MgNMgAl (u) 1.20 538 0.52 0.27 1.30 37.3 (NH ),AIF MgN 1.00 5913 0.21 0.041.20 35.6 (NHU AIF, 3.60 E 10A 005 0.22 0.01 36.1 MgAl (I) n.d."

13A 0.03 0.03 0.15 39.4 MgAl (1) nd 15A 0.13 0.44 0.01 37.8 MgA1(1) n.d.F Plant 0.16 0.34 0.50 32.2 0.90 Not identified .34 2 0.22 0.60 0.7734.0 0.90 None .33 3 0.40 0.80 1.10 33.0 0.40 None .38 4 0.46 0.71 1.1034.3 0.81 None .37 5 0.60 0.43 0.85 33.7 0.90 v. minor (NH )3AlF .39 60.27 0.45 0.68 35.0 0.70 v. minor (NHfl AlF .37 7 0.32 0.64 0.81 33.40.91 None .32

S.R. Wt.%F/wt.k MgO 3 wtfk A1,0, .33 will H O, in solution Ratio orthotopyrophosphate P,O, :30.

" Ratio orthoto pyrophosphate 1 ,0, 30:70

F added as (NH hSiF n.d. not determined because of uncertainty of smallconcentrations. Commercial fertilizer solutions.

' Ratio orthoto pyrophosphate P 0 :10; Fc O 0.5 wt. /i.

EXAMPLE IV solution as solid (NI-i.,) AlF and more slowly removevmagnesium as Mg(Nl-I (P O .6H O. These data are given to show that, whenconcentrations are not known precisely or are determined fromcompositions of thewet-process acid used for their production, it isbetter to have an excess of fluorine than a deficiency, since it wouldensure that any precipitated solids would be water soluble.

EXAMPLE V Effect of Insufficient fluoride The samples in Group E, TableI, were prepared initially to give sequestration ratios between 0.1 and0.3, but each mixture precipitated large amounts of MgAl(NH (P O F .6l-IO (I). The chemical analyses of these mixtures show that, under theseconditions, the amount of the least of the three impurities, Al O MgO,and P, will determine the amount of precipitate. The precipitate formedat S.R. of 0.3 or below cannot be removed by washing with water. Basedon the solidphase composition, a sequestration ratio of 0.2 would effectthe most nearly complete precipitation of all three constituents fromsolution, so that removal of these impurities by filtration should yielda more desirable, clarified product.

EXAMPLE VI Test on Commercial Products For this example, seven freshcommercial -34-0 products prepared from wet-process phosphoric acid weredivided into several portions to which increasing amounts of fluorinewere added, the mixtures were seeded and allowed to equilibrate 6 monthswith frequent agitation. With 3 days at the insufficient fluoridelevels, an increasing amount of MgAl(NH.,) P O F .6H O (I) was obtainedwith increasing fluoride content, showing that this precipitation isvery rapid when seeds are present. When fluorine concentrations were toohigh, the amount of solid (NHQ AIF increased with increasing fluorinecontent and the added seeds eroded and dissolved. Two samples of eachseries, which were in the optimum range of fluoridc concentrationremained clear or showed very minor precipitation. The sample with thelower fluorine content sometimes yielded only a small amount ofMgAl(NH.,) (P O F .6H O (l); the other sample deposited no precipitateand contained only the charged seeds; one mixture contained aninsignificant amount of an unidentified solid phase. The chemicalanalyses of these fully sequestered samples are shown by Group F. Since,in these mixtures, iron is the only other significant cation impuritythat may be complexed by '10 fluoride, consideration ofa term for ironwas necessary to maintain an S.R. of().4l at a polyphosphate content of50 percent; for example, suitable correlations could be obtained whenone-third of the weight percent Fe O was included in the equation.

Table l As indicated supra, I now prefer to use (2.5% A1 0 for thiscalculation.

These tests indicated that, for this type of fertilizer mixture, theamount of fluorine required to sequester iron is about one-ninth thatrequired for aluminum. It is to be recalled, however, that bothorthophosphate and pyrophosphate ions readily form complexes withironmore so than with aluminum. On the other hand, aluminum forms verystable, soluble complexes with fluorinemore so than iron. Thus, it mustbe conphates.

sults from the existence of iron in complexes with phos- Several othersludged commercial products have been clarified by the addition ofsoluble fluorides. Many of these were three-component fertilizer liquidsin which KC! had also been included, specifically 8-2- 5-3 and 7-21-7grades. The 8-25-3 had an abundant precipitate of MgAl(NH (P O F .6H O(I) (waterinsoluble form) and AI(NH P O OH.3H O and required an additionof 3 percent fluoride for clarification to a clear liquid. The 7-21-7likewise contained MgAl(Nl-l (P O F .6H O (I) and an equally abundantamount of an amorphous unknown gel phase which was misidentified as(Al,Fe)PO .nH O in the original application; 1.4 percent fluoride wasrequired to produce a clear liquid fertilizer from this mixture.

EXAMPLE Vll Effect of High Aluminum Contents Qualitative tests were madeon samples of commercial 10-33-0 and 8250, liquid fertilizers preparedfrom reclaimed phosphoric acid that had been used to pickle aluminum. itwas found that very small needle crystals of Al(NH P- O-,OH.3H Oprecipitated in these solutions and formed a badly gelled mixture thatwould not pour and could not be pumped from a railroad tank car. Theaddition of ammonium fluoride equivalent to approximately 2 percent Fwas made to this gel to provide an S.R. value of 0.4; within 20 hoursthe precipitate was gone and the mixtures were very fluid. in fact, thetreated material had higher clarity than most ammonium polyphosphateliquids prepared from wet-process phosphoric acid.

EXAMPLE Vll This example represents new data obtained subsequent to thefiling of my original application due to the discovery thatpolyphosphate was an important factor. For this example, a 1034-0 liquidfertilizer was simulated from reagent chemicals at a polyphosphate levelof 10 percent of the total P 0 with impurity levels of 1.0 percent MgO,1.0 percent Al O l .0 percent Fe O and 0.5 percent F. These cationimpurities represent near maximum values when compared to commercial10-34-0 liquid fertilizers produced from wet-process phosphoric acidsand the 0.5 percent fluoride represents the required S.R. value of 0.12necessary to sequester the MgO. In preparing this sample, it was obviousthat 10 percent polyphosphate was more than sufficient to sequester 1.0percent Fe,o,, which was charged as FePO, and rapidly dissolved, whereasthe 1 percent A1 was charged as MP0, and required 2 weeks fordissolution showing that percent polyphosphate is close to the requiredquantity for sequestering 1 percent Al O This sample was chosen from aseries of similar mixtures which included an increasing concentrationfor the impurities which also included 0.6 percent, 0.8 percent and 1.2percent, the first two of which rapidly became clear solutions and thelast one not only precipitated a significant quantity of MgAl(NI-I P O F-6I-I O but also contained undissolved Alwho have now been exposed to myteachings have come to appreciate that by practicing the instantinvention on fertilizer solutions containing lesser amounts of poly,several advantages, particularly in economics of operation andprocessing, are attained. Accordingly,

the emphasis in this example is on illustrating the effect of thepolyphosphate content on the proper value of the sequestration ratio.

To determine the effective range of S.R. (%F/%Mg0 2.5% AI O 0.33% Fe Oat various polyphosphate contents, three liquid fertilizers (10-34-0)were prepared from commercial wet-process phosphoric acid. Aliquots ofthese liquids were then treated with increasingly larger increments offluoride and subjected to the accelerated storage test as described inthe literature (Frazier, A. W. and Lee, R. G., Stabilizing LiquidFertilizers with Fluorine Compounds, Fertilizer Solutions," July-August1972). The three liquid fertilizers gave the following chemicalanalysis:

PO .nI-I O as charged. The absence of a significant precipitate in thissample shows that the pyrophosphate content needs to be approximatelyfive times the weight percent of Fe O A1 0 Therefore, this discoverythat pyrophosphate levels above this quantity for the purpose ofsequestering iron and aluminum represents luxurious consumption ofmaterials and also requires a higher fluoride level for sequesteringmagnesium, shows that the presently accepted value by the fertilizerindustry of percent as a minimum polyphosphate level is sufficient forapproximately 6 percent Al O Fe O which is far above the average sum of1 to 2 percent A1 0 Fe O and represents unnecessary expense.

EXAMP LE VIII Referring now more specifically to the FIGURE, as has beennoted above, the operable and optimum values for S.R. change as or areaffected by a change in the percent of the total P 0 in the solutionpresent as polyphosphate. It should be appreciated that in some of myearlier work emphasis was supplied for determining the operable andoptimum S.R.s for fertilizer formulations containing right at aboutpercent of the total P 0 present therein as polyphosphate, i.e.,nonortho acyclic species. This emphasis on 50 percent poly was dictatedin part by one school of thought dedicated to the proposition that suchammonium polyphosphate fertilizer solutions require at least this amountof the total P 0 present as poly in order to achieve the desiredsequestration characteristics relative to the impurities therein, whichimpurities are congeneric to the wet-process phosphoric acid used inultimately preparing such solutions. It should, of course, beappreciated that I do not wish to limit my invention only to solutionsof this polyphosphate level, as must be apparent from my discussion inExample 1, supra, particularly in its relationship to Group A of Table Isupra. This is particularly true in view of the somewhat changingattitude that such fertilizer solutions indeed need not necessarilycontain such great amounts of poly. Indeed, others skilled in the artSamples 1 and 2 were tested from 0.5 to 2.0 percent fluoride and sample3 from 0.0 to 1.2 percent fluoride. For all three samples a series wasobtained similar to those in the photograph of the 8th TVA demonstrationdisplay of Oct. 6 and 7, 1970, where those with insufflcient fluorideprecipitated abundant quantities of MgAl(NI-I (P O-,) F .6I-I O andthose with anexcess of fluoride precipitated the water-soluble salt(NI-I AlF The intermediate samples had no precipitate and were taken asthe solution compositions which would effectively describe the workableS.R. range at -each polyphosphate concentration. The results are shownbelow:

Effective %Mg0 fluoride Poly- 2.5% Al,0 concentration Calculatedphosphate +0.33% Fe O range S.R. range 82 2.07 0.9 to 1.7 0.44 to 0.8246 2.51 0.9 to 1.6 0.36 to 0.65 12 1.635 0.2 to 0.9 0.12 to 0.55

These results are shown in the FIGURE along with the originalrecommended value. The recommended value represents the minimum amountplus a safe excess to insure optimum results as indicated by line B. Theoperable range for any given percent poly level is limited by thevertical spacing defined between lines A and C, which lines A and C areshown as essentially straight line plots. As may be seen from theFIGURE, there exists a dependent and essentially straight-line typeproportional relationship between the percent of poly level in thesolutions and the maximum and minimum for the range of S.R. values. Onthe other hand, the relationship between the percent poly level and theoptimum S.R. that I recommend therefore does not appear to be a straightline function or plot possibly or perhaps because at lower percent polylevels my method of optimizing of the proper S.R. is a bit moresubjective in nature.

of magnesium, aluminum, iron, and mixtures thereof,

said impurities derived from said wet-process phosphoric acid andproviding cations of their dissolved salt solutions which give rise topost-precipitation of complexes therewith with the ammoniumpolyphosphates in said solutions, said ammonium polyphosphate solutionscontaining up to about 11 percent by weight nitrogen and 37 percent byweight phosphorus, expressed as P 0 said ammonium polyphosphatesolutions derived in whole or at least in part from the ammoniation ofwet-process phosphoric acid containing greater than 0.1 percentmagnesium and greater than 0.1 percent fluoride by a process comprisingthe addition to said ammonium polyphosphate solutions of a fluorinecompound, said fluorine compound selected from the group consisting ofsimple fluorides, fluosilicates, and mixtures thereof, and wherein theamount of fluorine added to said ammonium polyphosphate solutions forcomplexing principally the magnesium and aluminum therein to preventtheir precipitation is equivalent to a sequestration ratio of betweenabout 0.4 and about 0.5, said sequestration ratio being determined bythe empirical formula Sequestration Ratio (S.R.) Wt. F/Wt.% MgO 3 Wt%A1203 "i" Wt% F6203

1. A PROCESS FOR STABILIZING AMMONIUM POLYPHOSPHATE FERTILIZER SOLUTIONSCONTAINING ABOUT 50 PERCENT POLYPHOSPHATE LEVELS AGAIST THEPRECIPITATION OF PRINCIPALLY THE DISSOLVED IMPURITIES FROM THE GROUPCONSISTING OF MAGNESIUM, ALUMINUM, IRON, AND MIXTURES THEREOF, SAIDIMPURITIES DERIVED FROM SAID WET-PROCESS PHOSPHORIC ACID AND PROVIDINGCATIONS OF THEIR DISSOLVED SAT SOLUTIONS WHICH GIVE RISE TOPOSTPRECIPITATION OF COMPLEXES THEREWITH WITH THE AMMONIUMPOLYPHOSPHATES IN SAID SOLUTIONS, SAID AMMONIUM POLYPHOSPHATE SOLUTIONSCONTAINING UP TO ABOUT 11 PERCENT BY WEIGHT NITROGEN AND 37 PERCENT BYWEIGHT PHOSPHORUS, EXPRESSED AS P2O5, SAID AMMONIUM POLYPHOSPHATESOLUTIONS DERIVED IN WHOLE OR AT LEAST IN PART FROM THE AMMONIATION OFWET-PROCESS PHOSPHORIC ACID CONTAINING GREATER THAN 0.1 PERCENTMAGNESIUM AND GREATER THAN 0.1 PERCENT FLUORIDE BY A PROCESS COMPRISINGTHE ADDITION TO SAID AMMONIUM POLYPHOSPHATE SOLUTIONS OF A FLUORINECOMPOUND, SAID FLUORINE COMPOUND SELECTED FROM THE GROUP CONSISTING OFSIMPLE FLUORIDES, FLUOSILICATES, AND MIXTURES THEREOF, AND WHEREIN THEAMOUNT OF FLUORINE ADDED TO SAID AMMONIUM POLYPHOSPHATE SOLUTIONS FORCOMPLEXING PRINCIPALLY THE MAGNESIUM AND ALUMINUM THEREIN TO PREVENTTHEIR PRECIPITATION IS EQUIVALENT TO A SEQUESTRATION RATIO OF BETWEENABOUT 0.4 AND ABOUT 0.5 SAID SEQUESTRATION RATIO BEING DETERMINED BY THEEMPIRICAL FORMULA SEQUESTRATION RATIO (S.R.) = WT. % F/WT.% MGO + 3 WT%AL2O3 + 1/3 WT% FE2O3